This application is a National Phase of PCT/EP2004/011453, filed Oct. 13, 2004 and claims priority of German patent document DE 103 50 800.7, filed Oct. 29, 2003.
The present invention relates to a method for operating an internal combustion engine, in particular an auto-ignition internal combustion engine with direct injection.
In direct-injection internal combustion engines with auto-ignition, lean homogeneous fuel/air mixtures are often made to auto-ignite so that high efficiency levels and improved exhaust emissions are obtained. In such internal combustion engines which are referred to as HCCI or PCCI internal combustion engines, also referred to as internal combustion engines with spatial ignition combustion, a lean basic mixture of air, fuel and retained exhaust gas is generally formed at partial load and auto-ignited. In the case of full load, a stoichiometric mixture is frequently formed and spark-ignited because at high loads steep rises in pressure could occur in the combustion chamber due to the auto-ignition and these would adversely affect the operation.
DE 198 10 935 C2 discloses a method for operating an internal combustion engine which operates according to the four stroke principle and in which a homogeneous lean mixture of air, fuel and retained exhaust gas is formed and is burnt after compression ignition. In this context, there is an intermediate activation phase in order to expand the operating range of the motor with compression ignition. During the compression of the retained exhaust gas, an activation fuel quantity is injected into the combustion chamber and distributed as homogeneously as possible in the combustion chamber with the remaining components of the mixture. In this way thermal energy is supplied to the mixture by power and compression so that a chemical reaction or ignition is initiated in the vicinity of the top dead center of the charge change. The ignition time of the fresh charge can be controlled during the main compression by the time and the quantity of the activation injection.
According to the current state of the art, selective control of the combustion described above can be achieved only with difficulty since the time of auto-ignition depends very greatly on the parameters of the engine and the ambient conditions. For this reason, attempts are made to control the initiation of the compression ignition using suitable control variables, for example by way of a cylinder pressure signal. Such concepts are, however, associated with a high degree of expenditure on engine control technology which leads to a rise in the manufacturing costs of such internal combustion engines.
An object of the present invention is to provide a method for operating an internal combustion engine in which reliable operation with auto-ignition is ensured.
This object has been achieved by a method in which exhaust gas is retained in the combustion chamber of an internal combustion engine and is compressed during a charge change, a first fuel quantity being injected into the retained exhaust gas by direct fuel injection. A second fuel quantity is subsequently fed to the combustion chamber, preferably during the intake phase and/or in an initial part of the compression phase, so that a homogeneous fuel/air mixture is formed in the combustion chamber. In this connection, an auto-ignition time of the fuel/air mixture which is formed from the first and second fuel quantities is set as a function of a quantity ratio of the first fuel quantity to the second fuel quantity.
The injection of the first fuel quantity into the retained exhaust gas brings about optimum homogenization or preconditioning of the first fuel quantity, which leads to an increase in mixture reactivity of the fuel/air mixture which is formed from the first and second fuel quantities. This favors the inception of the auto-ignition, in particular at operating points with a low exhaust gas temperature. The first fuel injection is preferably performed between the closing of an outlet valve and the opening of an inlet valve. Depending on the injection time of the first fuel quantity, the preconditioning effect can extend beyond pure homogenization. If, in particular, the fuel is injected into the retained exhaust gas before the top dead center of the charge change, said exhaust gas also containing residual air, conversion-like reactions can occur, as a result of which the temperature of the mixture can be influenced, in particular increased.
In a refinement of the invention, the quantity ratio of the first fuel quantity to the second fuel quantity of 1:100 to 2:1, in particular of 1:5 to 1:3, is set. As a result the preconditioning effect can be adapted to the current operating point by the first fuel quantity. The injection of the second fuel quantity preferably takes place in synchronism with induction so that the reactivity of the mixture which is set by the first fuel quantity is neither increased nor decreased. The second fuel quantity is thus primarily used to set a desired load.
According to a further refinement of the invention, a center of gravity of the combustion is set by injecting a third fuel quantity, which is carried out after the injection of the second fuel quantity ends and preferably before a top dead center of the ignition. The third fuel quantity is aimed at reducing the reactivity of the total cylinder charge in particular under high loads. This is intended to reduce high burning speeds and large pressure rises in the combustion chamber.
In a further refinement of the invention, the period of combustion is set as a function of the third fuel quantity and its injection time. With the reduction in the reactivity of the mixture which is brought about by the third fuel quantity the burning through of the cylinder charge is slowed down so that, depending on the injection time of the third fuel quantity, the combustion period can be optimized as a function of the load.